EP0192035B1 - Process for the pre-esterification of free fatty acids in raw fats and/or oils - Google Patents

Abstract

A process for reducing the free fatty acid content of fats and oils by esterifying the free fatty acids with a lower monoalcohol in the presence of an acidic cation exchange resin as a solid esterification catalyst.

Description

The invention relates to a process for the pretreatment of triglycerides, in particular fats and / or oils of native origin, for the simplified removal of disruptive portions of free fatty acids in such starting materials. This pretreatment is intended in particular as a preliminary stage for a subsequent preparation of the native fatty substances, in which the triglycerides with lower monofunctional alcohols, in particular with methanol, are converted to glycerol and the corresponding fatty acid alkyl esters by transesterification which is known per se.

Fatty acid methyl esters are of great technical importance as a starting material for the production of fatty alcohols and other oleochemical products such as ester sulfonates, fatty acid alkanolamides and soaps. The industrial production of the fatty acid methyl esters takes place predominantly by catalytic transesterification (alcoholysis) of fatty acid triglyceride mixtures, as are present in the fats and oils of vegetable and animal origin.

Native fats and oils almost always contain considerable amounts of free fatty acids, whereby the content of free fatty acids - depending on the origin of the material and its history - can vary within a wide range and is almost always above 3 percent by weight.

Various processes are available for the transesterification of naturally occurring fatty acid triglycerides with alcohols. The choice of process conditions depends to a considerable extent on the amount of free fatty acids present in the triglycerides.

In the presence of alkaline catalysts, fats and oils can be transesterified at 25 to 100 ° C. with a 0.5 to 1.0 molar excess of alcohol under normal pressure to give the corresponding fatty acid ester mixtures. A corresponding method is described as the first stage of soap production in US Pat. No. 2,360,844. This alkali-catalyzed pressure-free transesterification can be carried out without any problems, as long as fats and oils are used which are largely free of water and whose free fatty acid content is below 0.5% by weight (corresponding to an acid number of about 1).

According to another process, fats and oils with a higher free fatty acid content can also be transesterified in the presence of alkali or zinc catalysts at 240 ° C. under a pressure of about 100 bar with a 7- to 8-molar excess of methanol to give the corresponding fatty acid methyl esters (Ullmann, Encyclopedia of Technical Chemistry, 4th edition, Volume 11 (1976), page 432).

Compared to the transesterification under increased pressure, the unpressurized transesterification is characterized by a significantly lower methanol requirement and - because of the lower reaction temperatures - by a low energy expenditure. In addition, the transesterification under normal pressure does not require expensive pressure reactors. Due to the fact that technical fats and oils almost always contain larger amounts of water and fatty acids. the pressure-free transesterification entails drying and a reduction in the acid number - e.g. by prior conversion of the free fatty acids into the corresponding alkyl or glycerol esters in the course of a pre-esterification reaction.

The pre-esterification of the acidic fats and oils can be carried out in the presence of alkaline catalysts at 240 ° C. and 20 bar (Ullmann. Encyclopedia of Industrial Chemistry. 4th Edition, Volume 11 (1976), page 432). This type of pre-esterification with methanol in turn requires the use of expensive pressure reactors.

It is also known to esterify the free fatty acids in oil with added monofunctional lower alcohols, especially methanol, in a homogeneous phase by means of acid catalysis, for. B. using p-toluenesulfonic acid as a catalyst. However, this process requires a relatively difficult catalyst removal with simultaneous water removal by washing the pre-esterified oil with methanol. In order to be able to separate the washing methanol with the water and catalyst, a liquid entrainer, which is essentially immiscible with the oil phase, must be added to the oil for the pre-esterification, with free glycerol preferably being used as the entrainer. A corresponding method is described in DE-OS 33 19 590. In this process, however, losses of free acid esters occur. Careful removal of the catalyst - usually p-toluenesulfonic acid - is absolutely necessary, since catalyst residues in the methyl ester can inhibit the hydrogenation catalysts during its subsequent conversion to the fatty alcohol.

GB-PS 782 480 describes a process for the preparation of diol or triol monocarboxylic acid esters, in which the diols or triols are reacted with one another in the presence of acidic ion exchange resins. The ion exchange resins can contain sulfonic acid, carboxylic acid or phosphonic acid groups as well as phenolic groups as polar groups. In the process, the raw materials polyol and carboxylic acid are reacted with one another. When the reaction is carried out, the water of reaction formed is continuously removed from the reaction mixture by azeotropic distillation with toluene. The deacidification of fats and oils is not addressed in the patent.

Bailey's Industrial Oil and Fat Products. Volume 2. Pages 124-126 mentions the use of ion exchange resins as esterification catalysts in connection with a multipurpose apparatus which is suitable for the various esterification and conversion processes to be carried out with fatty acids. From the construction of the apparatus (Figure 2.8 on page 125) with azeotropic storage, attached distillation column and It can be seen from the water separator that in the case of an esterification reaction the removal of the water of reaction by azeotropic distillation is provided. Deacidification of fats and oils is not mentioned here either.

The object of the invention is to develop a method. which includes all the advantages of an acid-catalyzed pre-esterification process, in particular working at comparatively mild temperatures and in the range of normal pressure, and is nevertheless free from the difficulties hitherto of reliably removing undesirable residual portions of the acid catalyst from the treated material. To achieve this object, the invention proposes to carry out the pre-esterification stage in the heterogeneous phase in such a way that acidic solid catalysts are used which cannot be dissolved in the product stream to be treated and which can consequently be reliably separated from the reaction mixture by simple phase separation.

The invention accordingly provides a process for reducing the content of free acids in fats and / or oils by treatment thereof with a 6 ₁ - ₄ mono alcohol in the presence of acidic esterification catalysts, which is characterized in that as catalyst solid cation exchanger resins in acidic form using and removes the water of reaction after separation of the reaction mixture from the cation exchange resin.

Preferred catalysts of the invention are strongly acidic cation exchange resins which contain free sulfonic acid groups bound to a polymer matrix. Corresponding ion exchange resins are known in various forms under numerous trade names. Corresponding types of resin with a macroporous structure which facilitate the intensive contact between the oil phase and the heterogeneous solid phase are particularly suitable according to the invention. Relevant references to these ion exchange resins with residues of strong acids as ion exchange groups - in particular sulfonic acid residues - can be found, for example, in Ullmanns Encyclopedia of Industrial Chemistry, 3rd edition, 8th volume. Munich-Berlin 1957, pages 806 to 817. There in particular Table 3, page 816.

Performing the pre-esterification of free fatty acids in the crude fats and / or oils according to the invention by means of heterogeneous solid catalysis on cation exchange resins combines the advantages of acidic catalysis described above with the absolutely reliable separability of the catalyst from the reaction mixture. This enables significant process simplifications and savings.

Suitable starting materials for the process according to the invention are native fats and / or oils which, as such, i.e. H. in the unpurified state, can be subjected to the process. Surprisingly, even in the continuous process after long-lasting process periods, there is no or no significant loss of activity on the heterogeneous solid catalysts. Any loss of activity that may occur can be eliminated by rinsing the catalyst phase, in particular with the monofunctional alcohol used for the pre-esterification, and if necessary by reactivating the acidic ion exchange groups in the resin.

Practically all fats and oils of vegetable or animal origin come into consideration as starting material for the process according to the invention, provided that their free fatty acid content is not inherently so low that they can be fed directly to the alkali-catalyzed unpressurized transesterification. Possible starting materials include in particular coconut oil, palm kernel oil, olive oil, rapeseed oil, cottonseed oil, lard oil, fish oil and beef tallow. The acid number of natural fats and oils and thus their free fatty acid content can vary widely. For example, the acid number of commercially available raw coconut oil is usually not above 20. For other vegetable oils, the acid number of good qualities is below 10; with lower qualities values from 20 to 25 can be observed. Technical tallow. which are valued and traded according to their acid number have - corresponding to a free fatty acid content of 1 to 20 percent by weight - acid numbers of 1 to 40, although occasionally higher values can also occur. In extreme cases, the acid number of the starting material for the process according to the invention can reach values of 60 and above.

(- ₄ alcohols, preferably methanol, C _') is esterified in the novel process of the present in the triglyceride mixtures of the starting material fraction of free fatty acids in the presence of the acidic cation exchange resins with existing excess monoalcohol. For this purpose, comparatively mild reaction conditions are chosen, so that a transesterification of the triglycerides with the monoalcohol does not take place or only to a limited extent. The ratio between triglyceride and the monoalcohol is expediently chosen so that, on the one hand, there is a clear excess of alcohol over the free acid to be esterified, and on the other hand, undisturbed workup is ensured at the end of the reaction. For this purpose, 10 to 50 volumes of C _' _ ₄ monoalcohol are normally used per 100 volumes of starting material.

The pre-esterification is usually carried out at normal pressure. Working with low overpressure can be advantageous in various cases. In this case, however, only pressures up to 5 bar come into consideration, for the realization of which no special pressure reactors are required.

The reaction temperature for carrying out the pre-esterification in the case of pressures in the range of normal pressure is in particular the temperature rich in the vicinity of the boiling point of the monoalcohol used, reaction temperatures between the boiling point and 10 ° C lower temperatures may be suitable. Accordingly, when working with methanol, temperatures in the range from 55 to 65 ° C. are preferred.

In order to remove the water of reaction which arises during the esterification and which would interfere with the subsequent transesterification of the oil to methyl ester, the reaction mixture separated from the cation exchange resin is subjected to drying before the circulating substream is recycled or the reaction mixture is processed further. The oil flow can, for example, be passed through a falling film evaporator. The water of reaction with the residual methanol passes into the vapor phase here. is condensed and thus removed from the process. A rectification of the alcohol / water mixture is possible, as a result of which the alcohol content is returned to the process cycle. However, the drying of the product stream is also possible in a manner known per se, for example the product stream drawn off from the solid catalyst can be passed over a drying agent - for example a molecular sieve - whereby the water of reaction is removed from the hydrocarbon mixture.

It has proven to be beneficial. To improve the flow and also the mass transfer conditions on the heterogeneous solid catalyst, add a diluent which is miscible with the reaction mixture. Suitable as diluent in the context of the invention are a recycled partial stream of the reaction mixture and / or a partial stream of a fatty acid alkyl ester, which can be branched off, for example, from the subsequent transesterification stage. The diluent is added to the mixture to be treated before it comes into contact with the solid catalyst. It can be added, for example, in amounts of 30 to 500 parts by volume of diluent to 100 parts by volume of starting material / alcohol mixture, in particular working with about 50 to 200 parts by volume of diluent to 100 parts by volume of starting material / alcohol mixture is preferred.

The heterogeneous solid cation exchange resins can be used in the process according to the invention in particular as a fixed bed through which the stream of liquid to be treated is passed. However, heterogeneous catalysis can also take place in a moving bed, for example in a fluidized bed and / or in a fluidized bed, provided that. that a reliable separation of the solid catalyst from the liquid phase is ensured by a subsequent separation step. Particularly suitable catalysts for the heterogeneous catalysis according to the invention are strongly acidic, macroporous, cation exchangers which can be used in anhydrous and nonpolar media, such as, for example, under the trademark Lewatit ( ^R ), in particular of the types SPC 118 BG, SPC 118, SPC 108 BG and SPC 108 are sold by Bayer AG.

With these solid catalysts, the crude oil is pumped, for example at a temperature of 60 to 65 ° C., together with methanol through a column with a fixed bed, which consists of catalyst resin. The acid number of the fats and / or oils is usually reduced to values of 1 or less. For example, when working with coconut oil, the acid number in crude oil can be reduced from 10 to below 1. In the preferred process, the catalyst volume, based on the oil throughput, is 1 to 10 1 resin per liter of crude oil per hour, it being possible in particular to work with ratios of 1.5 to 7.5 1 resin per liter of crude oil per hour. In practice, the pre-esterification and partial transesterification can be carried out, for example, in such a way that the liquid stream to be treated is passed through one or two ion exchange columns connected in series, which are heated to the reaction temperature. The liquid phase preferably flows through the ion exchange columns from bottom to top. Any gaseous components that arise - air bubbles or evaporated alcohol - pass through the catalyst bed in order to be condensed or blown off at the top of the column. Sieves at the top of the column prevent the discharge of the ion exchange resin. The sludge and mucilage contained in the crude oil separate from the pre-esterified oil. As described above, the product stream is freed from its water content and any residual free alcohol. A partial stream of the cleaned, dried, pre-esterified oil can be returned to the pre-esterification as a diluent.

Examples example 1

The pre-esterification was carried out continuously in two series-connected 100 mm diameter ion exchange columns, each filled with 3.5 1 Lewatit ^(R) SPC 118 BG ion exchange resin. Both jackets were heated to a temperature of 64 ° C via jacket heating.

Preheated to the reaction temperature of 64 ° C, 0.2 l / h of methanol mixed with 1 l / h of unpurified coconut oil with an acid number of 10, together with 0.98 l / h of circulating product, were pumped through the columns from bottom to top. In order to be able to return the circulating product free of water, the product stream emerging from the columns was passed through a falling film evaporator. which, as a circulation evaporator at ambient pressure, required a liquid phase temperature of 120 ° C. The anhydrous and freed from excess methanol oil had an acid number of 0.55 and a water content of 0.08% by weight.

The sludge and mucilage contained in the crude oil was separated in a separator. A partial stream of 0.98 of the product thus continuously pre-esterified was returned as a recycle stream. The remaining parts were fed to the transesterification.

Example 2

4.4 l of Lewatit ^(R) SPC 118 BG ion exchange resin were placed in a heatable tube with an inner diameter of 30 cm. 3 l / h of unpurified coconut oil with an acid number of 10 was mixed with 0.6 l / h of methanol and heated to a temperature of 64 ° C. in a pass through a heat exchanger.

After flowing through the fixed bed at a temperature of 62 ° C. once, the mixture had an acid number of 0.56.

Example 3

The Lewatit ^(R) SPC 108, SPC 118 and SPC 118 BG ion exchangers were investigated in a comparative test. A mixture of 1800 ml coconut oil, 900 ml MeOH and 460 ml methyl ester was pumped through a heated column (64 ° C.) with 517 g resin in each case at a circulation rate of 25 l / h.

The mixture contained a proportion of free fatty acids at the start of the reaction. The acid number was 3.2.

After pumping over for 2 hours, the mixtures after the column had acid numbers of 0.91, 1.15 and 0.89. After 6 hours of pumping over, the acid numbers were 0.32, 0.42 and 0.41.

Claims (12)

1. A process for reducing the content of free fatty acids in fats and/or oils by treatment thereof with a C_'-₄ monoalcohol in the presence of acidic esterification catalysts, characterized in that solid cation exchanger resins in acidic form are used as the catalyst and the water of reaction is removed after separation of the reaction mixture from the cation exchanger resin.

2. A process as claimed in claim 1, characterized in that strongly acidic cation exchanger resins containing free sulfonic acid groups on a polymer matrix are used.

3. A process as claimed in claims 1 and 2, characterized in that native fats and/or oils are subjected to the process in non-pretreated form.

4. A process as claimed in claims 1 to 3, characterized in that the C_'-₄ monoalcohols are used in quantities of 10 to 50 parts by volume per 100 parts by volume starting material.

5. A process as claimed in claim 4, characterized in that methanol is used as the monoalcohol.

6. A process as claimed in claims 1 to 5, characterized in that a diluent is passed over the cation exchanger resin together with the mixture of starting. material and alcohol, a circulated partial stream of the reaction mixture and/or a partial stream of a fatty acid alkyl ester being used as the diluent.

7. A process as claimed in claims 1 to 6, characterized in that the reaction mixture separated from the cation exchanger resin is subjected to.drying before the circulated partial stream or the reaction mixture is further processed.

8. A process as claimed in claims 1 to 7, characterized in that it is carried out under pressures around normal pressure and at temperatures around the boiling point of the monoalcohol.

9. A process as claimed in claim 8, characterized in that, where methanol is used, the process is carried out at temperatures in the range from 55 to 65°c.

10. A process as claimed in claims 1 to 9, characterized in that the diluent is added in quantities of 30 to 500 parts by volume to 100 parts by volume of the starting material/alcohol mixture.

11. A process as claimed in claim 10, characterized in that the diluent is added in quantities of 50 to 200 parts by volume to 100 parts by volume of the starting material/ alcohol mixture.

12. A process as claimed in claims 1 to 11, characterized in that it is carried out until the acid value of the fats and/or oils has fallen to values of 1 or lower.